thank you for this simple solution, it's a bit curious that the lossless setting produces lower quality than high, isn't it?

Like you proposed to do I reduced the resolution of images using PS before insertion to Powerpoint. I tested this procedure for some png images but for those inserted in higher resolution (downscaled in powerpoint) the export result (swf) looks better than the ones reduced in advance. In your answere you reported just the opposite, does this procedure maybe depend on filetype or else? Is the Presenter player able to take advantage of feeding in higher resolved images for situations where the exported presentation is displayed on bigger screens?

Presenter will scale to fit any screen it is displayed on. low res images may show some pixelization when displayed on large, high resolution monitors.

In the end, it comes down to what you are trying to achieve. The settings and quality of images used, will need to be set up for your intended audience. I usually stick with JPG images, as they are consistently reliable. So my experience with PNG images is limited.

As to the lossless button, it is curious. Looking back at the documentation, it looks like the Lossless setting is intended to be better than the Low setting.

From Presenter 8 documentation:

High

The largest file size and highest quality image.

Medium

The best balance between file size and image quality.

Low

The smallest file size and lowest quality image.

Lossless

(available in Adobe Presenter 7.0.7 and later) High quality images that are embedded in SWF files. The size of SWF files is more than the ones that result from the ‘Low’ option.